1H NMR Investigation of the Composition, Structure, and Dynamics of

cyclohexane-d12 (nongel forming) by variable-temperature 1H NMR spectra, T1, and T2 relaxation measurements. All of the NMR data are consistent with ...
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H NMR Investigation of the Composition, Structure, and Dynamics of Cholesterol-Stilbene Tethered Dyad Organogels Dean C. Duncan and David G. Whitten* Los Alamos National Laboratory, Materials Sciences and Technology Division, Center for Materials Science, Los Alamos, New Mexico 87501 Received February 7, 2000. In Final Form: April 28, 2000

The self-assembly of stilbene-cholesterol tethered dyads into fibrous networked structures induces the thermoreversible gelation of several classes of organic solvents. Herein, the gel and solution phases of the gelator, p-R-cholesteryl-p′-octoxystilbenoate (1), are investigated in 1-butanol-d10 (gel forming) and cyclohexane-d12 (nongel forming) by variable-temperature 1H NMR spectra, T1, and T2 relaxation measurements. All of the NMR data are consistent with detection of 1 in the gel phase by a fast exchange on the NMR time scale between residual 1 dissolved in the gel liquid and 1 in a “mobile” (NMR detectable) gel solid. This conclusion is significant since in previous NMR studies on other gel systems detection of gelator in the gel phase was considered only within “mobile” regions of the gel solid. The decrease in peak integrals of 1 in the gel phase by declining temperatures reflects a convolution of (1) a decrease in concentration of 1 dissolved within the gel liquid as it condenses onto the gel solid strands and (2) a transition in the gel solid from “mobile” to more “rigid” (NMR silent) regions. The relaxation measurements provide evidence for the existence of a sol aggregate of 1 in 1-butanol-d10 observed above the temperature required for gel formation, Tg. This aggregate is likely the incipient precursor structure to the gel, but its aggregate length is not yet long enough to nucleate the growth of the interpenetrating solid network required for solvent gelation. The absence of regioselectivity in the temperature dependence of the 1H relaxation rate behavior of 1 along the gel f sol f solution trajectory implies involvement of both the stilbene and cholesterol substructures of 1 in forming the sol and gel aggregates. However, one caveat is that, in the gel phase, the internal phenyl ring of the stilbene moiety of 1 may have greater mobility than in other regions of 1.

Introduction The self-assembly of low molecular weight molecules into entangled supramolecular structures as is required for the effective gelation of solvents1 (organogels in nonaqueous solvents and hydrogels in aqueous solutions) remains attractive for a host of new applications including the synthesis of templated materials and heterogeneous catalysts2-6 and in molecular recognition,7 sensor fabrication,8 and laser media9 among others. These gels are distinct from covalently linked polymeric gels in that only noncovalent interactions sustain the gelator aggregate framework required for gel formation. At the gelation (1) For recent reviews on organogels see: (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. (b) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1630-1643. (2) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. Chem. Commun. 1997, 543. (3) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675-12676. (4) Hafkamp, R. J. H.; Kokke, B. P. A.; Danke, I. M.; Geurts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545-546. (5) Sohna Sohna, J.-E.; Fages, F. Chem. Commun. 1997, 327-328. (6) (a) Jung, J. H.; Ono, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1999, 7, 1289-1291. (b) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. Chem. Lett. 1999, 10, 1119-1120. (c) Ono, Y.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Lett. 1999, 1, 23-24. (d) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477-1478. (7) (a) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Ishi-i, T.; Yoshihara, K.; Shinkai, S. J. Org. Chem. 1999, 64, 2933-2937. (b) Shinkai, S.; Murata, K. J. Mater. Chem. 1998, 8, 485-495. (c) James, T. D.; Kawabata, H.; Ludwig, R.; Murata, K.; Shinkai, S. Tetrahedron 1995, 51, 555-566. (8) (a) Li, S.; John, V. T.; Irvin, G. C.; Rachakonda, S. H.; Mcpherson, G. L.; O’Connor, C. J. J. Appl. Phys. 1999, 85, 5965-5967. (b) VelascoGarcı´a, N.; Valencia-Gonza´lez, M. J.; Dı´az-Garcı´a, M. E. Analyst 1997, 122, 1405-1409. (9) Lal, M.; Pakatchi, S.; He, G. S.; Kim, K. S.; Prasad, P. N. Chem. Mater. 1999, 11, 3012-3014.

temperature, Tg, the self-assembled supramolecular aggregate structures, e.g., reverse micelles among others, reach a critical length at which they become entangled forming a fibrous network that encapsulates and retains solvent via capillary forces. These gels are thermally reversible and exhibit viscoelastic liquidlike or solidlike rheological behavior. Several different types of molecular species that facilitate organogel formation have been identified, which include various amphiphiles,1,10 cholesterol derivatives,1,6,11-13 amino acid variants,1,14 metal complexes,1,15 calix[n]arenes,1,16 and modified carbohydrates.1,17,18 The derivatization of cholesterol at the C(3) position with aromatic hydrocarbons (e.g., anthracene, anthra(10) (a) Terech, P. Colloid Polym. Sci. 1991, 269, 490. (b) Zhou, Z.; Georgalis, Y.; Liang, W.; Li, J.; Su, R.; Chu, B. J. Colloid Interface Sci. 1987, 116, 473. (c) Murdan, S.; Gregoriadis, G.; Florence, A. T. Int. J. Pharm. 1999, 183, 47-49. (11) (a) Jeong, S. W.; Murata, K.; Shinkai, S. Supramol. Sci. 1996, 3, 83-86. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Angew. Chem. Int. Ed. 2000, 39, 1862-1865. (12) Geiger, C.; Stanescu, M.; Chen, L. H.; Whitten, D. G. Langmuir 1999, 15, 2241-2245. (13) (a) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Intl. Ed. 1996, 35, 1324 and references therein. (b) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558 and references therein. (14) (a) Hanabusa, K.; Okui, K.; Karaki, K.; Kimura, M.; Shirai, H. J. Colloid Interface Sci. 1997, 195, 86-93. (b) Hanabusa, K.; Naka, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1994, 26832684. (c) Bhattacharya, S.; Acharya, S. N. G. Chem. Mater. 1999, 11, 3121-3132. (15) Bremi, J.; Brovelli, D.; Caseri, W.; Ha¨hner, G.; Smith, P.; Tervoort, T. Chem. Mater. 1999, 11, 977-994. (16) Aoki, M.; Nakashima, K.; Kawabata, H.; Tsutsui, S.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1993, 347. (17) (a) Beginn, U.; Keinath, S.; Mo¨ller, M. Macromol. Chem. Phys. 1998, 199, 2379-2384. (b) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. J. Org. Chem. 1999, 64, 412-426.

10.1021/la0001631 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/07/2000

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quinone, and azobenzene) provides entry to an important class of molecules for the gelation of organic liquids.1,6,11-13 These molecules are efficient gelators presumably because competition between two distinct types of noncovalent intermolecular interaction, cholesterol-cholesterol and aromatic-aromatic, leads to spatial anisotropy in the kinetics of gelator aggregation growth. In an effort to illuminate further the aggregate structures responsible for organogel formation within this class of gelators, one of us recently reported the synthesis and characterization of a series of tethered trans-stilbene/cholesterol dyad gelators, where the stilbene moiety served both as a necessary structural feature for organogel formation and as a spectroscopic probe that is sensitive to aggregation phenomena.12 These molecules gel organic solvents at concentrations less than 0.1 wt % in gelator and the compound, p-R-cholesteryl-p′-octoxystilbenoate (1), was found to be the most efficient gelator in this series. Moreover, on the basis of both the photophysical behavior and the spectrosocopic signatures of the trans-stilbene moiety of 1 within different gelled solvents, it was concluded that stilbene-stilbene interactions are significant in these organogel phases and a model for the microstructure of the gel aggregate networks was proposed (vide supra). This paper addresses further the microstructure of these organogel aggregates as is provided by high-resolution 1 H NMR spectroscopy and both longitudinal and transverse magnetic relaxation rate measurements, 1/T1 (LMR) and 1/T2 (TMR), respectively. Few NMR studies of organogels exist owing to the considerable line broadening observed in gel phases.19-21 In these studies, information on the gelator aggregate structures is obtained from detection of regioselective changes in both line widths and chemical shifts of NMR active nuclei upon gel phase formation. This report further clarifies the nature of observable resonances in the NMR spectra of low molecular weight gelators in their gel phases using conventional high-resolution NMR spectrometers. Additionally, a theoretical basis for interpreting NMR relaxation of low molecular weight gelators within gel phases is outlined. Experimental Section Details on the synthesis of 1 are reported elsewhere.12 Deuterated solvents were obtained from Cambridge Isotopes at the highest deuteration levels available (cyclohexane-d12, 99.9+% D; 1-butanol-d10, 98+%) and stored under an anhydrous N2 atmosphere in Schlenk tubes over activated 4 Å molecular sieves. The NMR organogel samples were prepared within tared 5 mm o.d. NMR tubes by measuring the weight difference of added 1 followed by delivering a known volume of deuterated solvent by syringe. The solvent volume was minimized (450-500 µL) to reduce thermal gradients within the sample during NMR (18) (a) Yoza, K.; Ono, Y.; Yoshihara, K.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Commun. 1998, 907-908. (b) Amanokura, N.; Yoza, K.; Shinmori, H.; Shinkai, S.; Reinhoudt, D. N. J. Chem. Soc., Perkin Trans. 2 1998, 2585-2591. (c) Amanokura, N.; Kanekiyo, Y.; Shinkai, S.; Reinhoudt, D. N. J. Chem. Soc., Perk. Trans. 2 1999, 10, 1995-2000. (d) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Kiyonaka, S.; Hamachi, I.; Shinkai, S. Chem. Lett. 1999, 3, 225-226. (e) Takeuchi, M.; Yoda, S.; Chin, Y.; Shinkai, S. Tetrahedron Lett. 1999, 40, 3745-3748. (f) Yoza, K.; Amanokura, N.; Ono, Y.; Akao, T.; Shinmori, H.; Takeuchi, M.; Shinkai, S.; Reinhoudt, D. N. Chem. Eur. J. 1999, 5, 2722-2729. (19) (a) Capitani, D.; Rossi, E.; Segre, A. L. Langmuir 1993, 9, 685689. (b) Capitani, D.; Segre, A. L.; Dreher, F.; Walde, P.; Luisi, L. L. J. Phys. Chem. 1996, 100, 15211-15217. (20) (a) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809-3817. (b) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464-9470. (21) Buyse, K.; Berghmans, H.; Bosco, M.; Paoletti, S. Macromolecules 1998, 31, 9224-9230.

Figure 1. Temperature dependence of the 1H NMR spectra of 1 in 1-butanol-d10 (12.6 mM; Tg ) 56.5 °C). See Chart 1 for proton assignment key; X ) solvent resonance. (a) Stilbene protons A, B, C, D, E, and -O-CH2-; cholesterol protons CH(6) and CH(3): a, 65.4 °C; b, 60.9 °C; c, 56.5 °C; d, 52.0 °C; e, 47.4 °C; f, 34.0 °C; g, 25.0 °C. (b) Cholesterol protons ang-CH3(18), ang-CH3(19), chain CH3(21), and chain CH3 (26, 27): a, 65.4 °C; b, 60.9 °C; c, 56.5 °C; d, 47.4 °C; e, 34.0 °C; f, 25.0 °C. measurement while still maintaining both good shimming and high-resolution spectra. The tubes were sealed and protected from ambient light while heating gently until 1 dissolved completely. For quantifying changes in NMR peak integrations of 1 as a function of temperature, a coaxial tube, containing a known concentration of p-dioxane solution, was inserted into the tube containing 1 and resealed prior to the onset of gelation. The resulting solutions were cooled to ambient temperature in the dark and allowed to equilibrate for at least 15 h prior to NMR measurements.

Cholesterol-Stilbene Tethered Dyad Organogels

Langmuir, Vol. 16, No. 16, 2000 6447 Chart 1

The NMR spectra and relaxation measurements were recorded on a Varian Gemini 2000 instrument equipped with a 4.74 T magnet (200 MHz 1H) and a variable-temperature accessory. The probe temperature was calibrated between 25 and 100 °C using an ethylene glycol standard. Variable-temperature NMR measurements were recorded at successively higher temperatures with a minimum sample equilibration time of 30 min after the targeted probe temperature was reached. The T1 measurements were performed using the inversion recovery pulse sequence. The T2 measurements were performed using the CPMG pulse sequence with dephasing times short enough (0.4 ms) to suppress unwanted J-modulation of the spin-echoes.22,23 For both T1 and T2 relaxation measurements, a minimum of 16 points was obtained. The resulting curves could be fit satisfactorily to single exponential decays for all measurable proton groups and at all recorded temperatures.24

Results 1H

NMR Spectral Behavior. The temperature-dependent 1H NMR spectral changes of 1 in 1-butanol-d10 are shown in Figure 1 along with the assignments of key resonances examined in both the stilbene and cholesterol substructures. These assignments are labeled with respect to the structure of 1 as is illustrated in Chart 1. All of the stilbene resonances could be monitored, including the alkyl ether methylene protons, -O-CH2-. However, residual protiated solvent did obscure some of the resonances in the aliphatic region, particularly the alkyl chain and the alicylic cholesterol ring protons. Nevertheless, the cyclic CH(6) and CH(3) protons and the “angular” methyl protons, ang-CH3(18) and ang-CH3(19), were sufficiently dispersed to provide signals diagnostic of the cholesterol substructure. At ambient temperature, the 1-butanol-d10 gel shows small peaks for the stilbene aromatic protons with partially resolved J-couplings. The -O-CH2- peak is broad with unresolved J-coupling, and the cholesterol resonances are obscured by the residual solvent protons. However, as the temperature increases, the upfield shift (22) Freeman, R.; Hill, H. D. W. In Dynamic Nuclear Magnetic Resonance Spectroscopy; Jackman, L. M., Cotton, F. A., Eds.; Academic Press: New York, 1975; Chapter 5. (23) Larive, C. K.; Rabenstein, D. L. Magn. Res. Chem. 1991, 29, 409-417. (24) Fits to a single-exponential function yielded R2 g 0.998 for all data sets. Clear double exponentiality in the resonance decays of 1 were observed only when they were convoluted with nearby solvent resonances (e.g., alkyl chain and cholesterol cyclic methylene groups). These second components were identified unambiguously to solvent relaxation. The successful fitting to single exponential decays was surprising, particularly for the methyl groups, which are usually nonexponential as a consequence of both free methyl and overall molecular rotations. (25) The photophysical and spectroscopic signatures of the transstilbene substructure in 1 indicate no aggregation of 1 in cyclohexane.

of the solvent hydroxyl proton resonance reveals the CH(6) and CH(3) protons, and the ang-CH3(18) and ang-CH3(19) protons become detectable from the solvent background by increases in their intensities. All resonances of 1 sharpen, and their intensities grow as the temperature is increased until no further changes are observed above the gelation temperature, Tg ) 56.5 °C. Furthermore, no significant changes in chemical shifts are observed and no new peaks are detected. To decouple the peak intensity and line width changes from any instrumental broadening in the gel phase, the peaks were integrated as is shown in Figure 2. Consistent with the observed intensity and line width changes, the peak integrals of 1 increase with increasing temperature and reach a maximum at 4.5 °C above Tg. Unfortunately, the temperature-dependent changes observed on progressing from the gel to the solution phase, in both the spectral appearances and peak integrals, do not permit any clear distinction between the behaviors of the cholesterol and stilbene substructures. This behavior is in contrast with the NMR behavior reported on other molecular-based gel systems.19-21 The spectra of 1 in cyclohexane-d12, a solvent in which neither gelation nor aggregation of 1 occurs,25 show no significant temperature dependence, aside from small line width changes attributed to the variation in solution viscosity. 1 H Nuclear Magnetic Relaxation Behavior of 1. The longitudinal and transverse magnetic relaxation rate constants (LMR ) 1/T1 and TMR ) 1/T2) were measured

Figure 2. Temperature dependence of the normalized 1H NMR peak integrations of 1 in 1-butanol-d10 (12.6 mM; Tg ) 56.5 °C). Stilbene protons: A (O), B + C (0), E (]), D (×), -O-CH2- (4). Cholesterol protons: CH(3) (2).

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Figure 3. Temperature dependence in the 1H longitudinal and transverse relaxation rates of 1 in 1-butanol-d10 (12.6 mM; Tg ) 56.5 °C, straight lines) and in cyclohexane-d12 (12.6 mM; nongel forming; dashed lines). (a) and (c): stilbene A (O), B (0), C (]), D (×), E (4), -O-CH2- (b). (b) and (d): cholesterol CH(6) (O), CH(3) (0), ang-CH3(19) (]), ang-CH3(18) (×).

for 1 both in 1-butanol-d10 and in cyclohexane-d12 as a function of temperature as is shown in Figure 3. The LMR and TMR rate behavior for each proton resonance could be fit satisfactorily to a single-exponential decay at each temperature. In the absence of either aggregation or specific solvent interactions, the temperature dependence of the LMR and TMR rates of 1 should be comparable in 1-butanol-d10 and cyclohexane-d12 since they have similar viscosity temperature dependencies (vide infra). Consequently, the relaxation rate behavior in cyclohexane-d12 models that which would be observed in 1-butanol-d10 in the absence of 1 aggregation or gel formation. As a visual aid for comparison, the cyclohexane-d12 data are normalized to those in 1-butanol-d10 at 65.4 °C, a temperature high enough to preclude any significant aggregation of 1 in 1-butanol-d10 (Tg ) 56.5 °C). The nearly perfect extrapolation of the normalized data in cyclohexane-d12 to the higher temperature experimental points in 1-butanol-d10 (T > 65.4 °C) adds further justification for this comparison. The absolute differences between relaxation rates observed in these two solvents at temperatures above Tg are attributed to absolute differences in solvent viscosity and the smaller deuteration content in 1-butanol-d10. Positive deviations in relaxation rate behavior between the 1-butanol-d10 and normalized cyclohexane-d12 data are taken as evidence for aggregation of 1, as is detected by increases both in intermolecular 1H-1H dipolar interaction densities and in motional restrictions (vide infra). A. 1H Longitudinal Magnetic Relaxation (LMR) of 1. The slopes in the plots of LMR rate constant vs reciprocal temperature (Figure 3a,b) show small increases for the stilbene protons between 65.4 °C and Tg in

1-butanol-d10 relative to those in cyclohexane-d12. However, as the temperature decreases below Tg, the slopes for the stilbene protons B, C, E, and -OCH2 in 1-butanold10 flatten and eventually cross over the corresponding curves in cyclohexane-d12. The slopes for protons A and D in 1-butanol-d10 show similar behavior to each other but do not cross over their corresponding curves in cyclohexane-d12. In the cholesterol region, protons CH(6), CH(3), and ang-CH3 (19) in 1-butanol-d10 show a small prompt increase in LMR rate constants between 65.4 °C and Tg relative to those in cyclohexane-d12, whereas the ang-CH3(18) protons show a small anomalous decrease. This is followed by the formation of well-defined maximum LMR rate constants at either Tg or a slightly lower temperature in 1-butanol-d10, which, in turn, is followed by sharp decreases in LMR rate constants as the temperature decreases further for all four proton groups. These marked decreases in LMR rate constants lead to strong deviations and crossings between the slopes, d(LMR)/ d(T-1), in 1-butanol-d10 gel and cyclohexane-d12 solution. B. 1H Transverse Magnetic Relaxation (TMR) of 1. In plots of TMR rate constants vs reciprocal temperature (Figure 3c,d) the stilbene protons C, D, E, and -OCH2in 1-butanol-d10 vs cyclohexane-d12 all show small prompt rate increases just below 65.4 °C that proceed monotonically to Tg. For these protons in 1-butanol-d10, no discernible differences are apparent in their slopes, d(TMR)/ d(T-1). Proceeding from Tg toward ambient temperature, these protons all show large monotonic increases in TMR rate constants with similar slopes, in 1-butanol-d10 relative to cyclohexane-d12. In contrast, for protons A and B, the small monotonic increases in TMR rate constants in

Cholesterol-Stilbene Tethered Dyad Organogels

1-butanol-d10 extend from 65.4 to 47.0 °C and the large monotonic increases in TMR rate constants occur between 47.0 °C and ambient temperature. The slopes in these plots for protons A and B are both similar to each other and similar to the other stilbene protons, C, D, E, and -OCH2. The cholesterol resonances, CH(3), CH(6), angCH3(18), and ang-CH3(19) behave similarly. Plots of TMR rate constants vs reciprocal temperature show small monotonic increases in TMR rate constants between 65.4 °C and Tg in 1-butanol-d10 relative to cyclohexane-d12, followed by large monotonic increases in TMR rate constants between Tg and ambient temperature. The large errors in these measurements do not permit any meaningful comparison in slopes of these plots. Nevertheless, they do appear similar within error. The regioselectivity observed in the temperature-dependent TMR rate constants for stilbene protons A and B in the gel phase relative to all other protons in 1, is consistent with greater mobility in this phenyl ring of stilbene relative to other regions of 1 in the gel (vide infra). The LMR and TMR rates of the solvent are insensitive to the solution f gel phase change, which indicate little change in solvent viscosity as the gel is formed. Discussion 1H NMR Behavior. Any inference on

Synopsis of the the structure and/or dynamics of 1 in the gel phase must account for the following NMR behaviors as is observed between 25 °C to well over Tg. (1) Beginning near Tg, the peak integrals for all measured protons decline as the temperature decreases with similar temperature dependencies. (2) All peaks begin to broaden near Tg and then broaden further as the temperature decreases. (3) No new resonances are detected. (4) No significant changes in chemical shifts are observed. (5) Both LMR and TMR processes for all measured protons exhibit single exponential decays. (6) The LMR rate constants show little temperature dependencies with similar behaviors for all protons. Near and below Tg, these temperature dependencies are different from those observed in cyclohexane-d12, a solvent in which aggregation of 1 does not occur. (7) The TMR rate constants in 1-butanol-d10 show strong temperature dependencies with a small discontinuity 9 °C above Tg followed by a strong, sharp discontinuity at Tg (except for stilbene protons A and B which show it at 9 °C below Tg). However, these discontinuities are not observed in cyclohexane-d12. (8) Both the LMR and TMR rate constants of the solvent in the gel phase reflect the bulk viscosity of the neat liquid. 1H NMR Relaxation Theory Pertinent to Gel Formation. To deduce from the NMR behavior any information on the phase composition, structure, and dynamics of 1 in gel-forming solvents, it is instructive to derive a general theoretical expression that relates, qualitatively, the relaxation rate temperature dependencies of a solute to changes in its environment. The dominant NMR relaxation pathway for solute protons at low field strengths is mediated by magnetic 1H-1H dipolar interactions.26,27 These include both intramolecular and intermolecular contributions. The intermolecular com(26) Abragam, A. In Principles of Nuclear Magnetism, 2nd ed.; Clarendon Press: Oxford, 1983. (27) Canet, D.; Robert, J. B. In NMR at Very High Field; Vol. 25 in the series NMR Basic Principles and Progress; Robert, J. B., Guest Ed.; Springer-Verlag: Berlin, 1991; Vol. 25, Chapter 3.

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ponent to 1H-1H dipolar relaxation in 1 is expected to contain contributions from both self-interactions (aggregates) and solvent interactions, although the latter contribution has been eliminated as much as possible by using anhydrous solvents containing the highest levels of deuteration commercially available. Under nonaggregating conditions and omitting any solute-solvent 1H-1H dipolar interactions, the expression for the relaxation rate constant, Ri, of proton i in 1 is illustrated in eq 1

(r-6 ∑ ij ‚Jij(ω,T)) j*i

Ri ) Kγ2

(1)

where K is a collection of constants for the dipolar interaction, γ is the magnetogyric ratio for hydrogen, rij is the intramolecular distance between coupled proton spins, i and j, Jij(ω,T) is the temperature-dependent spectral density at the coupled nuclei, and the sum runs over all spins j that are coupled to spin i.27 The only differences in the expressions between LMR and TMR rate constants are in the forms of the spectral density terms, which are identical except for additional lowfrequency components in TMR. The spectral density arises from magnetic field fluctuations generated by molecular motions within the vicinity of the dipolar-coupled nuclei and serves to modulate the dipolar interaction. Consequently, any observed temperature dependence in relaxation rate behavior of a nonaggregated solute is directly related to changes in solute dynamics. Provided that the solute molecular structure and the solute/solvent interactions are known with sufficient accuracy, the spectral density term can be calculated explicitly. However, since the analysis presented in this paper is qualitative in nature, no attempt is given here to calculate the explicit spectral density terms for 1 either in solution or in aggregate states. For 1 in a single aggregate phase, the expression for the relaxation rate is illustrated in eq 2 where the two

Ri ) Kγ2(

(r-6 ∑ ij ‚Jij′(ω,T)) + ∑ (Jij′′′(rij′,ω,T))) j*i j′ intra

(2)

inter

terms reflect intramolecular and intermolecular contributions, respectively. The intermolecular term is summed over spins j′ residing on neighboring solute molecules that are dipolar coupled to the observed spin i. The dipolar separation, rij′, is time dependent because of translational diffusion, and the expanded explicit expression for the compact intermolecular spectral density term, Jij′′′(rij′,ω,T), must reflect this contribution in the time-dependent correlation function prior to Fourier transformation.28 The intramolecular spectral density terms of the nonaggregate and aggregate states of 1, Jij(ω,T) and Jij′(ω,T), respectively, are differentiated because of the more restricted motions of 1 in the aggregate. Furthermore, the spectral densities of the intramolecular and intermolecular terms for the aggregate state, Jij′(ω,T) and Jij′′′(rij′,ω,T), respectively, are different because of significant translational motion contributions present in Jij′′′(rij′,ω,T) that are absent in Jij′(ω,T). In general, the “line-narrowing” regime is obtained when ω is much greater than the Larmor precession frequency, ω0. Here the spectral densities in both LMR and TMR rate expressions are identical, therefore the rates are identical, and narrow resonances are obtained as is typical for NMR behavior in nonviscous (28) For further details on the form of this function, see: Cowan, B. In Nuclear Magnetic Resonance and Relacation; Cambridge University Press: Cambridge, 1997; pp 238-242.

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liquids. However, as ω approaches and decreases below ω0, as is observed in solids and in viscous liquids, the weighting of the spectral density in TMR to low-frequency components becomes manifest by a divergence in LMR and TMR rate behavior. The LMR rate constant is maximized when ω ∼ ω0 with an overall inverted parabolictype shape in plots of LMR rate vs J(ω). However, when ω e ω0, the TMR rate constants continue to increase resulting in severely broadened lines that become undetectable under conventional high-resolution measuring conditions. Interpretation of the NMR Behavior of 1 in Both Gel and Liquid Solution Phases. Observation (1) is consistent with the formation of solid 1 in the gel phase in which the motion of 1 is so restricted that it becomes undetectable under our NMR measuring conditions. This structure, hereafter referred to as the “rigid” gel solid region, appears near Tg and its mole fraction increases as the temperature decreases (Figure 2). However, observation (1) does not preclude the existence of “mobile” gel solid regions that could be NMR detectable under our experimental conditions. Such mobility could be created by solvent-induced “swelling” of domains within the gel solid. The question still remains as to what is the origin of the resonances of 1 that are detected in the gel phase? In previous reports on other gel systems, the observation of high-resolution spectra in the gel phase is attributed to detection of gelator within “mobile” gel solid regions.19,20 In fact, the possibility of detecting residual gelator in the liquid solution part of the gel is not mentioned. This point is crucial to the NMR interpretation of gel structure and dynamics and is considered further below. Observations (5), (6), and (7), together with the relaxation rate expressions for nonaggregate and aggregate behavior of 1, eqs 1 and 2, demonstrate, for two reasons, the detection of a fast exchange on the NMR time scale between nonaggregated 1 in liquid solution and 1 in an aggregate, regardless of whether the aggregate is “mobile” (NMR detectable) or “rigid” (NMR silent). From thermodynamic considerations, both nonaggregate and aggregate structures of 1 must coexist over some range of temperatures. The aggregates of 1 to be considered include colloidal structures within a liquid phase (sol), which should be detectable in high-resolution NMR spectra and both “rigid” (NMR silent) and “mobile” (NMR detectable) structures within a fibrous solid network (gel). The following two reasons pertain to 1 in a gel-forming solvent within a temperature interval where both nonaggregate and aggregate structures of 1 coexist. First, if only the nonaggregate of 1 were detectable by NMR, and no exchange with aggregate structures were to occur on the NMR time scale, then the temperature dependence in LMR and TMR rate constants of 1 would be described by eq 1. Consequently, similar behavior would be anticipated in a solvent in which aggregation of 1 does not occur. This is true because the spectral density in the two solvents, J(ω,T), would be similar provided that they have comparable bulk viscosities and that there are no significant solute-solvent specific interactions, e.g., hydrogen bonding. Observation (8) indicates that the liquid part of the gel phase has a similar viscosity to that of the pure bulk solvent. Therefore, the large differences in relaxation behavior of 1 observed between a gel-forming and a non-gel-forming solvent, observations (6) and (7), argue against the above supposition. Clearly, if 1 were detectable only within a nonaggregate structure, then the observed LMR and TMR rate behaviors must reflect the existence of an aggregate structure by an exchange process

Duncan and Whitten

that is fast on the NMR time scale. The relaxation rates would reflect an average of contributions from eq 1 and eq 2 producing single-exponential decay behavior as is consistent with observation (5). Second, if 1 were NMR detectable in both nonaggregate and aggregate structures (with identical chemical shifts as is consistent with observation (4)), then a fast exchange of 1 between structure types must occur because, in accord with eq 1 and eq 2, a slow exchange would produce biexponential LMR and TMR decay behavior which is in conflict with observation (5).29 Consequently, regardless of whether aggregates of 1 produce detectable NMR peaks, the LMR and TMR rate constants do reflect an average of contributions from eq 1 and eq 2 producing single-exponential decay behavior as is consistent with observation (5). A general expression for this behavior is illustrated in eq 3 where the contributions from nonaggregate and aggregate structures are weighted by their respective temperature-dependent mole fractions, χ(T), and, for generality, q different aggregate moieties are considered.

(r-6 ∑ ij ‚Jij(ω,T)))solution + j*i

Ri ) Kγ2‚{(χS(T) q

(r-6 ∑ χp(T)( ∑ ij ‚Jij′(ω,T)) + p)1 j*i intra

∑j′ Jij′′′(rij′,ω,T))aggregate p}

(3)

inter

and q

χS(T) +

∑ χp(T) ) 1

p)1

Further information on the nature of the aggregates that are in exchange with 1 in solution can be obtained by close inspection of the temperature dependence in the NMR relaxation behavior of 1. In 1-butanol-d10, the divergence in behavior between the LMR and TMR rate constants of 1 at temperatures eTg, in addition to the formation of an inverted parabolic-type shape in plots of LMR rate vs reciprocal temperature (Figure 3), are consistent with a transition from the “line-narrowing” regime in the solution phase, i.e., ω . ω0, to one where ω approaches and dips below ω0. This change in spectral density is manifest in a large decrease in motional freedom of 1 either by its self-aggregation or by a large change in solvent viscosity. However, both the LMR and TMR rates of the solvent do not show any large changes as the gel is formed, observation (8), which indicates that the gel liquid has a viscosity similar to the neat solvent. Therefore, the increased motional restriction of 1 is attributed to the formation of aggregates, which must be in fast exchange with 1 in the nonaggregated state as is consistent with observations (4) and (5) and is described by eq 3. Note that not all protons in 1 reflect a well-defined maximum LMR rate near Tg since two different proton groups i and k will have different spectral densities, Ji(ω) and Jk(ω), with corresponding differences on approaching the condition, ω ∼ ω0. Over the temperature interval, Tg < T < Tg + 6 °C, the small inflections in both LMR and TMR rate constants observed in 1-butanol-d10 relative to those in cyclohexane(29) Facile detection of a fast component (gel solid) in the presence of a slow component (gel liquid solution) is expected.

Cholesterol-Stilbene Tethered Dyad Organogels

Figure 4. Temperature-dependent partitioning and dynamical exchange of 1 between nonaggregated and aggregated states as deduced from 1H NMR spectroscopic and relaxation measurements: (A) liquid solution; (B) optically transparent colloid; (C) gel. Tg ) gelation temperature; (NMR - d) ) NMR detectable; (NMR - s) ) NMR silent.

d12 are attributed to the formation of colloidal aggregates of 1, i.e., a sol, that are in fast exchange with nonaggregated 1 in solution (Figure 4B). In this temperature region, the aggregates do not exceed the critical length required to nucleate the growth of an interpenetrating network as is necessary to induce solvent gelation. It is likely that this aggregate is the incipient precursor to the gel solid structure, 1-agg(ip), which is reflected in short correlation lengths as is typical of globally amorphous glassy solids.30 Over the temperature interval, 25 °C < T < Tg, the large inflections in LMR rate constants observed in 1-butanol-d10 relative to those in cyclohexane-d12, reflect the formation of the gel phase where 1 is located within solid fibrous networks as is detected by either of two mechanisms or both. (a) Detection of residual 1 dissolved in the gel liquid that is in rapid exchange with 1 in the gel solid. Exchange with either “mobile” (NMR detectable) or “rigid” (NMR silent) gel solid regions are considered for now. (b) Detection of 1 only within “mobile” gel solid regions. Any distinction between these two cases must take into account observation (4), the absence of any substantial chemical shift changes as the gel is formed, which is puzzling. Indeed, significant spectral shifts are expected for 1 within a densely packed solid, which would be reflected by any fast exchange with 1 in solution. In contrast, for solid gel regions that are accessible to solvent, smaller shifts would be anticipated and such regions would also have considerable freedom of motion for 1. Consequently, case (a) is modified to include only exchange with “mobile” gel solid regions. Since both 1 dissolved in the gel liquid and 1 in the gel solid must coexist over some temperature interval of T e Tg, case (a) must be operative over this temperature range as is consistent with observation (5). In summation, these arguments maintain the existence of both “rigid” and “mobile” regions of 1 within the gel solid; however, only 1 in the “mobile” gel solid region exchanges with 1 dissolved in the gel liquid on the NMR time scale. Consequently, the resonances observed in the gel phase are a convolution of both residual 1 dissolved in the gel-liquid solution and 1 in “mobile” (30) Zarzycki, J. In Glasses and Amorphous Materials; Vol. 9 in the series Materials Science and Technology; VCH: Weinheim and New York, 1991; Vol. 9.

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Figure 5. Structure and dynamical exchange between 1 and its aggregates (cross-sectional views), 1-agg(p) and 1-agg(ip), produced within the interval Tg < T e Tg + 6 °C (sol). The “primary” unit aggregate, 1-agg(p), is generated by core-centered stacking of the cholesterol substructures into columnar helical arrays with stilbenes on the periphery (∼6 nm diameter,